ECE 333 Renewable Energy Systems

Lecture 7: Power System Operations, Wind as a Resource

Prof. Tom Overbye

Dept. of Electrical and Computer Engineering

University of Illinois at Urbana-Champaign

overbye@illinois.edu

Announcements

• Start reading Chapter 7; also read Prof. Sauer's article on course website explaining reactive power

• HW 3 is posted; it will be covered by an in-class quiz on Thursday Feb 13– Material from Power Systems history and operations will be

covered on exams (such as true/false)

2

Power Flow

• A common power system analysis tool is the power flow – It shows how real and reactive power flows through a network,

from generators to loads

• Solves sets of non-linear equations enforcing "conservation of power" at each bus in the system (a consequence of KCL)– Loads are usually assumed to be constant power – Used to determine if any transmission lines or transformers are

overloaded and system voltages

• Educational version PowerWorld tool available at– http://www.powerworld.com/gloversarmaoverbye

3

PowerWorld Simulator Three Bus System

Bus 2 Bus 1

Bus 3Home Area

204 MW

102 MVR

150 MW

150 MW 37 MVR

116 MVR

102 MW 51 MVR

1.00 PU

-20 MW 4 MVR

20 MW -4 MVR

-34 MW 10 MVR

34 MW-10 MVR

14 MW -4 MVR

-14 MW

4 MVR

1.00 PU

1.00 PU

106 MW 0 MVR

100 MWAGC ONAVR ON

AGC ONAVR ON

Load with

green

arrows

indicating

amount

of MW

flow

Used

to control

output of

generator Direction of arrow is used to indicate

direction of real power (MW) flow

Note the

power

balance at

each bus

4

Area Control Error (ACE)

•The area control error is the difference between the actual flow out of an area, and the scheduled flow.•Ideally the ACE should always be zero.•Because the load is constantly changing, each utility must constantly change its generation to “chase” the ACE.

https://www.misoenergy.org/MarketsOperations/RealTimeMarketData/Pages/ACEChart.aspx

MISO ACE|(in MW) from 9/19/12. Atthe time the MISO loadwas about 65GW

5

Automatic Generation Control

• BAs use automatic generation control (AGC) to automatically change their generation to keep their ACE close to zero.

• Usually the BA control center calculates ACE based upon tie-line flows; then the AGC module sends control signals out to the generators every couple seconds.

6

Three Bus Case on AGC

Bus 2 Bus 1

Bus 3Home Area

266 MW

133 MVR

150 MW

250 MW 34 MVR

166 MVR

133 MW 67 MVR

1.00 PU

-40 MW 8 MVR

40 MW -8 MVR

-77 MW 25 MVR

78 MW-21 MVR

39 MW-11 MVR

-39 MW

12 MVR

1.00 PU

1.00 PU

101 MW 5 MVR

100 MWAGC ONAVR ON

AGC ONAVR ON

7

Generator Costs

• There are many fixed and variable costs associated with power system operation.

• The major variable cost is associated with generation.• Cost to generate a MWh can vary widely.• For some types of units (such as hydro and nuclear) it

is difficult to quantify.• Many markets have moved from cost-based to price-

based generator costs

8

Economic Dispatch

• Economic dispatch (ED) determines the least cost dispatch of generation for an area.

• For a lossless system, the ED occurs when all the generators have equal marginal costs.

IC1(PG,1) = IC2(PG,2) = … = ICm(PG,m)

9

Power Transactions

• Power transactions are contracts between areas to do power transactions.

• Contracts can be for any amount of time at any price for any amount of power.

• Scheduled power transactions are implemented by modifying the area ACE:

ACE = Pactual,tie-flow - Psched

10

100 MW Transaction

Bus 2 Bus 1

Bus 3Home Area

Scheduled Transactions

225 MW

113 MVR

150 MW

291 MW 8 MVR

138 MVR

113 MW 56 MVR

1.00 PU

8 MW -2 MVR

-8 MW 2 MVR

-84 MW 27 MVR

85 MW-23 MVR

93 MW-25 MVR

-92 MW

30 MVR

1.00 PU

1.00 PU

0 MW 32 MVR

100 MWAGC ONAVR ON

AGC ONAVR ON

100.0 MW

Scheduled 100 MW

Transaction from Left to Right

Net tie-line flow is now

100 MW 11

Security Constrained Economic Dispatch

• Transmission constraints often limit system economics.

• Such limits required a constrained dispatch in order to maintain system security.

• In three bus case the generation at bus 3 must be constrained to avoid overloading the line from bus 2 to bus 3.

12

Security Constrained Dispatch

Bus 2 Bus 1

Bus 3Home Area

Scheduled Transactions

357 MW

179 MVR

194 MW

448 MW 19 MVR

232 MVR

179 MW 89 MVR

1.00 PU

-22 MW 4 MVR

22 MW -4 MVR

-142 MW 49 MVR

145 MW-37 MVR

124 MW-33 MVR

-122 MW

41 MVR

1.00 PU

1.00 PU

0 MW 37 MVR100%

100%

100 MWOFF AGCAVR ON

AGC ONAVR ON

100.0 MW

Dispatch is no longer optimal due to need to keep

Line from bus 2 to bus 3 from overloading 13

Multiple Area Operation

• If Areas have direct interconnections, then they may directly transact up to the capacity of their tie-lines.

• Actual power flows through the entire network according to the impedance of the transmission lines.

• Flow through other areas is known as “parallel path” or “loop flows.”

14

Seven Bus Case One-line Diagram

Top Area Cost

Left Area Cost Right Area Cost

1

2

3 4

5

6 7

106 MW

168 MW

200 MW 201 MW

110 MW 40 MVR

80 MW 30 MVR

130 MW 40 MVR

40 MW 20 MVR

1.00 PU

1.01 PU

1.04 PU1.04 PU

1.04 PU

0.99 PU1.05 PU

62 MW

-61 MW

44 MW -42 MW -31 MW 31 MW

38 MW

-37 MW

79 MW -77 MW

-32 MW

32 MW-14 MW

-39 MW

40 MW-20 MW 20 MW

40 MW

-40 MW

94 MW

200 MW 0 MVR

200 MW 0 MVR

20 MW -20 MW

AGC ON

AGC ON

AGC ON

AGC ON

AGC ON

8029 $/MWH

4715 $/MWH 4189 $/MWH

Case Hourly Cost 16933 $/MWH

System has

three areas

Area left

has one

bus Area right has one bus

Area top

has five

buses

15

Seven Bus Case: Area View

Actual

flow

between

areas

Loop flow can result in higher losses

Area Losses

Area Losses Area Losses

Top

Left Right

-40.1 MW

0.0 MW

0.0 MW

0.0 MW

40.1 MW

40.1 MW

7.09 MW

0.33 MW 0.65 MW

System has

40 MW of

“Loop Flow”

Scheduled

flow

16

Seven Bus System – Loop Flow?

Area Losses

Area Losses Area Losses

Top

Left Right

-4.8 MW

0.0 MW

100.0 MW

0.0 MW

104.8 MW

4.8 MW

9.44 MW

-0.00 MW 4.34 MW

100 MW Transaction

between Left and Right

Note that

Top’s

Losses have

increased

from

7.09MW to

9.44 MW

Transaction has actually decreased

the loop flow

17

Pricing Electricity

• Cost to supply electricity to bus is called the locational marginal price (LMP)

• Presently PJM and MISO post LMPs on the web• In an ideal electricity market with no transmission

limitations the LMPs are equal• Transmission constraints can segment a market,

resulting in differing LMP• Determination of LMPs requires the solution on an

Optimal Power Flow (OPF)

18

Three Bus Case LMPs: Line Limit NOT Enforced

Bus 2 Bus 1

Bus 3

Total Cost

0 MW

0 MW

180 MWMW

10.00 $/MWh

60 MW 60 MW

60 MW

60 MW120 MW

120 MW

10.00 $/MWh

10.00 $/MWh

180 MW120%

120%

0 MWMW

1800 $/hr

Line from Bus 1 to Bus 3 is over-loaded; all buses have same marginal cost

Gen 1’s

cost

is $10

per

MWh

Gen 2’s

cost

is $12

per

MWh

19

Three Bus Case LMPS: Line Limits Enforced

Bus 2 Bus 1

Bus 3

Total Cost

60 MW

0 MW

180 MWMW

12.00 $/MWh

20 MW 20 MW

80 MW

80 MW100 MW

100 MW

10.00 $/MWh

14.01 $/MWh

120 MW 80% 100%

80% 100%

0 MWMW

1921 $/hr

Line from 1 to 3 is no longer overloaded, but now

the marginal cost of electricity at 3 is $14 / MWh 20

Generation Supply Curve

0

20

40

60

80

0 10000 20000 30000 40000

Generation (MW)

Pri

ce (

$ /

MW

h)

Base Load

Coal and Nuclear

Generation

Natural

Gas Generation

As the load goes up so does the price

Renewable Sources Such as Wind Have Low Marginal Cost, but they are Intermittent 21

MISO LMPs on Sept 19, 2012 (11:50am EST which is CDT)

Available on-line at https://www.misoenergy.org/LMPContourMap/MISO_All.html22

MISO LMPs on Feb 6, 2015, 1pm Central

23Available on-line at https://www.misoenergy.org/LMPContourMap/MISO_All.html

MISO Annual Load Duration Curves

24https://www.misoenergy.org/Library/Repository/Report/Annual%20Market%20Report/2013%20Annual%20Market%20Assessment%20Report.pdf

MISO Average Prices and Wind Output

https://www.misoenergy.org/Library/Repository/Report/Annual%20Market%20Report/2013%20Annual%20Market%20Assessment%20Report.pdf 25

Wind Power Systems

Photos taken Kate Davis near Moraine View State Park, IL 26

Historical Development of Wind Power

• The first known wind turbine for producing electricity was by Charles F. Brush turbine, in Cleveland, Ohio in 1888

http://www.windpower.org/en/pictures/brush.htm

• 12 kW• Used electricity

to charge batteries in the cellar of the owner’s mansion

Note the person

27

Historical Development of Wind Power

• First wind turbine outside of the US to generate electricity was built by Poul la Cour in 1891 in Denmark

• Used electricity from his wind turbines to electrolyze water to make hydrogen for the gas lights at the schoolhouse

http://www.windpower.org/en/pictures/lacour.htm 28

Historical Development of Wind Power

• In the US - first wind-electric systems built in the late 1890’s

• By 1930s and 1940s, large numbers in rural areas not served by the grid for pumping water and sometimes electricity generation

• Interest in wind power declined as the utilitygrid expanded and as reliable, inexpensive electricity could be purchased

• Oil crisis in 1970s created a renewed interest in wind until US government stopped giving tax

• Renewed interest again since the 1990sPhoto: www.daviddarling.info/encyclopedia/W/AE_wind_energy.html 29

Global Installed Wind Capacity

Source: Annual Market Update 2013, Global Wind Energy Council,

Total worldwide electric capacity is 4500GW, sowind, at almost 250GW, is 5.6% of total

30

Wind Capacity Additions by Region

Source: Annual Market Update 2013, Global Wind Energy Council, 31

Top 10 Countries - Installed Wind Capacity (as of the end of 2013)

Source: Annual Market Update 2013, Global Wind Energy Council, 32

US Wind Resources

http://www.windpower.org/en/pictures/lacour.htmhttp://www.windpoweringamerica.gov/pdfs/wind_maps/us_windmap.pdf 33

US Wind Capacity by State, 12/31/14

34

Wind Map for Illinois at 80m

35

Worldwide Wind Resource Map

Source: www.ceoe.udel.edu/WindPower/ResourceMap/index-world.html36

Types of Wind Turbines

• “Windmill”- used to grind grain into flour or pump water

• Many different names - “wind-driven generator”, “wind generator”, “wind turbine”, “wind-turbine generator (WTG)”, “wind energy conversion system (WECS)”

• Can have be horizontal axis wind turbines (HAWT) or vertical axis wind turbines (VAWT)

• Groups of wind turbines are located in what is called either a “wind farm” or a “wind park”

37

Vertical Axis Wind Turbines

• Darrieus rotor - the only vertical axis machine with any commercial success

• Wind hitting the vertical blades, called aerofoils, generates lift to create rotation

• No yaw (rotation about vertical axis) control needed to keep them facing into the wind

• Heavy machinery in the nacelle is located on the ground

• Blades are closer to ground where windspeeds are lowerhttp://www.absoluteastronomy.com/topics/Darrieus_wind_turbine 38

Horizontal Axis Wind Turbines

• “Downwind” HAWT – a turbine with the blades behind (downwind from) the tower

• No yaw control needed- they naturally orient themselves in line with the wind

• Shadowing effect – when a blade swings behind the tower, the wind it encounters is briefly reduced and the blade flexes

39

Horizontal Axis Wind Turbines

• “Upwind” HAWT – blades are in front of (upwind of) the tower

• Most modern wind turbines are this type• Blades are “upwind” of the tower• Require somewhat complex yaw control to keep

them facing into the wind– Need to search for the wind to start turning

• Operate more smoothly and deliver more power• Largest turbines are on the order of 6 MW with 1.5

MW a quite common design

40

Number of Rotating Blades

• Windmills have multiple blades– need to provide high starting torque to overcome weight of the

pumping rod– must be able to operate at low wind speeds to provide nearly

continuous water pumping– a larger area of the rotor faces the wind– Note, most seem to write “wind speed” as two words

• Turbines with many blades operate at much lower rotational speeds - as the speed increases, the turbulence caused by one blade impacts the other blades

• Most modern wind turbines have two or three blades

41

Worldwide Wind Energy Company Market Share, 2013 Installations

Source:http://www.statista.com/statistics/272813/market-share-of-the-leading-wind-turbine-manufacturers-worldwide/42

Vestas Stock Price

https://uk.finance.yahoo.com/echarts?s=VWS.CO#symbol=VWS.CO;range=my43